No, he's referring to the synths. Some synths came with direct serial ports that could interface directly to a Mac that had a serial connection, similar to how you find synths now with USB ports so that they can interface directly to a computer with USB. If your synth doesn't have one of these other than the MIDI ports, then you would need a MIDI interface with a serial connection that would connect to that Mac. I have an old one that I use with my Quadra called the MidiMan miniMacMan (MidiMan is now known as M-AUDIO). I also have an M-AUDIO MidiSport USB 2x2 interface for use with my iMac G5.
You can find schematics for building your own 1x1 MIDI interface on the web, but you can still find some of the Mac-serial MIDI interfaces on eBay and from some vendors (even some with multiple MIDI ins and outs).